Thermocouple offsets are errors

[Cookerr]

Hi there! I'm currently working on a project that deals with temperature controlled water and have been testing T type, grounded tip, thermocouples on a V700 with V200-18-E4XB snap-in I/O. I've been getting some results which I'm struggling to reconcile and was wondering if anyone had some insight...

I recognized there is was offset between actual temperature and the thermocouple reading. I could add this offset in logic but after testing across my measured range (15 degree C to 60 degree C), I noticed the offset is not linear. When measuring low end temps (20 degree C), there is about a 5 degree C offset, and by the time you reach 40 degree C this has been reduced to about 2 degree C. A simple answer may be to program a different offset for different ranges but that seems like a weak solution that I would not feel confident with. I understand that there is about a 30 min cold junction warm up time and all my data was collected well after that window. I also understand that thermos do have a limit to their accuracy but I don't think I'm at the point where I'm asking to much of them yet.

I've heard of people using thermocouple transmitters which can be tuned and then sent to an analog input but that seems like a waste since the PLCs already have direct T/C reading capabilities,  but maybe this is the only way to get the accuracy I want?

Lastly, I read in the instruction manual that every attached thermocouple adds 100ms to my read time and I just want to make sure I'm understanding this correctly - if I were to use 4 T/Cs my read time is going to be 400ms? And if this is the case I wondering if there's any way to improve this.

All of this is relatively new waters for me so any help with the above issues would be greatly appreciated!

Thanks,

Alex/

[Joe Tauser]

Your offset is most likely being cause by a ground potential difference between where the thermocouples are located and where the PLC is installed.  The -E4XB is not isolated and if there are any VFDs in the system the grounded end of the thermocouple will pick up noise like an antenna. 

Remember that the output from a thermocouple is very small - for a type T at 20 C it's 0.790 mV.  That's 0.00079 volts.  It doesn't take much to upset them.

The first thing I would do is go to an ungrounded thermocouple, which will slow down your response time a bit but will help with the noise issue.

Speaking of response time, why are you concerned with a 400 ms response on a thermocouple?  Do you know what response you actually need?  It takes 100 ms to blink your eyes.  Response time is usually determined as a function of the ratio of the mass of the media being heated (or cooled) and the available power of device used to change its temperature such as heating elements or a chiller.

Also, why type T?  This type is typically used for really low temperature applications such a refrigeration - type J and K are much more common as they have higher usable linear temperature ranges.

 

Joe T.

[Pavel_setnicar]

Also , are yours termocouples connected directly to the inputs of PLC or there are some extension cables between?

There should be no other joints except on the PLC input.

[viscoelastic]

if you are running your wire in cable trays as we usually do here at work, then make sure your tray has a metal separator between sensor wires and motor wires.  We have went as far as to pull our sensor wires through flexible metal conduit and lay that in the tray. That step alone solves 90% of our phantom current issues.